Portal for custom dimension always spawns on nether roof?

I have an issue with my custom dimension, which is nether type generation.

Let's say we are in the overworld, and I make a portal to my custom dimension (let's call this the vether)

If I make the portal (while I'm in the overworld) on the surface, that portal will take me to the roof of the vether.

But if I make the portal underground, the portal will spawn in the underground vether like its supposed to.

Same thing happens when I travel back to the overworld, underground portals (so anywhere in the vether besides the roof) will take me to the underground overworld, and portals on the nether roof take me to the surface.

I know this is a problem with the two dimension correlating underground and above ground, (maybe including x and y variables somehow)

But does anyone know a solution to this issue? If so I would really appreciate it.